Composite silica article and method of fabricating same



Get. 10, 1933. E THOMSON 1,930,327

COMPOSITE SILICA ARTICLE AND METHOD OF FABRICATING SAME Filed May 10.1932 I Fig. l.

Inventor: Elihu Thomson,

His Attorney."

Patented Oct. 10, 1933 UNITED STATES PATENT OFFICE COMPOSITE SILICAARTICLE AND METHOD OF FABRICATING SAME New York Application May 10,1932. SerialNo. 610,385

4 Claims.

The present invention relates to the fabrication of massive articles,such, for example, as large telescopic mirrors, from a material such assilica which, because of its refractory nature, cannot be cast in afused state by ordinary methods used in fabricating glass articles. Itis the object of my invention to provide an improved method forfabricating such articles.

It has been customary to fabricate silica bodies of extensive surface byproviding first a base of sintered silica, formed by heating a mass ofsand or comminuted silica by radiant heat in an electric resistancefurnace, and then depositing thereon a layer of fused, structurelesssilica by projecting particles of silica through a flame on this baseand causing them to unite by fusion. A process for carrying out theflame-deposition of silica is described in United States Letters Patent1,869,- 163, Niedergesass, patented July 26, 1932 and also in UnitedStates application Serial No. 453,362,

Ellis and Winckler, filed May 17, 1930 (see corresponding French Patent718,857), both applications being assigned to the same assignee as thatof the present application.

The process of building up masses of silica by the progressive fusionand incorporationof particles of silica projected through a flame willbe referred to herein briefly as an accretion process.

As described in a copending application Serial No. 610,382, filed May10, 1932 by Alvarado L. R. Ellis and Gunnar A. F.'Winckler an entirecomposite article, of which an astronomical mirror is an example, isformed by the accretion process, a base being sprayed at a relativelyhigh rate and a facing layer of silica being deposited thereon at alower rate in order to produce greater density and transparency in thefacing. While this duplex spraying process can be carried out in theorder named, certain difficulties are encountered. The spraying of thebase causes ridges to appear on the surface of the silica mass duringspraying. As the spraying continues these ridges become more pronouncedinstead of being smoothed out by .the flame. Due to the lessened heatingof the sides of the ridges, bubbles appear in the imperfectly fusedsilica onthese sides. Furthermore, any impurities which are accidentallyintroduced into the mass of silica during the spraying of the base,forexample, by particles of fire-brick or the like dropping on the silicafrom the roof of the furnace in which the operation is carried out or byparticles of metal or oxide becoming dislodged from the sprayingapparatus, cause the trapping of bubbles in the sprayed product whichpersist throughout the sprayed basein the form of so-called runners. Aseach layer is deposited the deposition of silica upon one of theserunners causes the bubbles to continue through the material being laiddown. If such runners are present, the facing layer of transparentsilica will be imperfect to that extent. In any event the ridges must beremoved before the clear layer is deposited. As this involves coolingthe base to room temperature loss of time and additional expense isinvolved.

In accordance with my present invention these diiiiculties are overcomeby spraying the composite body in reverse order, that is, by sprayingthe facing first and the relatively thick base or support subsequently.

My invention will be described in greater detail in connection with theaccompanying drawing in which Fig. 1 is a vertical section of thesupport and a clear layer of silica in the process of being depositedthereon, the burner being symbolically indicated; Fig. 2 is a verticalsection illustrating the second stage of fabrication wherein a secondlayer of translucent silica is deposited upon the initial layer oftransparent silica; Fig. 3 is a modification of my invention wherein thearticle is given a predetermined contour while being deposited on asupport; and Fig. 4 is a somewhat diagrammatic vertical section of aheating furnace in which the silica deposition is carried out.

As illustrated in Fig. 1, a layer of silica 4 is deposited upon asupport 5 consisting of suitable refractory material, as, for example amixture of about 90 to 95 parts comminuted silica and about 10 to 5parts ball clay, the proportions not being critical. The clay binder maybe omitted under some conditions. This layer 5 rests on a support offire brick 6. Particles of silica are caused to impinge upon the support5 by the burner 7 and are fused together by the heat of the flame.Repeated traverse of the burner over the initial layer of silica causesa desired layer of transparent silica to be built up. The burner issupplied with combustible and combustionsupporting gases (hydrogen orthe like) through the conduits 8, 9 and with finely divided silicaconveyed by one of the gases from a feeding device which is not shown.Separate conduits 10, 11 are shown to convey cooling water to theburner. The deposition of the silica occurs in a suitable furnace (seeFig. 4) comprising an enclosure 12, containing a suitable heating meansas conventionally indicated by the electric heaters 13, 14. As thedetails of construction of the burner and other parts of the apparatusand the method of supplying finely divided silica are described in theabove-mentioned United States patent application and patent (and theforeign patent above identified corresponding in disclosure thereto) theparticular features of this apparatus and process will not be repeatedin the present applications The rate of feed of the silica powder andthe gas and the rate of traverse of the flame are regulated to producethe desired density and transparency in the product.

After the clear layer 4 has been deposited, the rate of feed of thesilica preferably is materially increased, thereby causing correspondingincrease in the rate of deposition of the silica but resulting in thedeposition of silica layer 15 which may be less transparent, may containgas inclusions and is of a lower density than the transparent silica. Asthis lower density layer eventually forms the backing layer when thearticle produced is to be used for supporting the reflecting surface inastronomical mirrors, this inclusion of gas bubbles and resultingnon-transparency of the silica is of no disadvantage. On the other hand,advantages accrue due to the increased rate of deposition. of therelatively thick layer 15, and its lesser weight per unit of volume. Forexample, for deposition of a high grade, transparent layer 4, a rate offeed of silica may be employed with a given set of conditions, such asthe size and shape of the burner, the rate of gas feed, rate of traverseof burner and so forth, whereby cubic inch of silica is deposited by thecombustion of about ten cubic feet of hydrogen, or other suitablecombustionsupporting gas, this deposition occurring 'ata rate requiringabout one hour. 1

If this relatively slow rate of deposition were continued during thefabrication of the entire article, it would not only greatly increasethe cost of the desired article but also devitrification of the silicamight occur in the long heating period resulting in destruction of theresulting article. If, on the other hand, the rate of deposition isincreased ten timesor more, then devitrification will not occur, but onthe other hand the silica deposited under the conditions will not beclear and free from gas bubble inclusions. For a base or backing layerthis is of no disadvantage. For practical purposes a cheaper grade ofsilica also may be employed for the backing layer than for the face. 4

As shown in Fig. 3, the initial deposition of the article during theaccretion process with a configuration roughly similar to the finaldesired configuration results in less grinding and is less tedious andexpensive. Referring to this figure, a layer 16 is first formed upon asupport 1'7, which has an external configuration corresponding to thedesired configurationof the layer 16 which in the completed article willform the external layer. The layer 17 comprises a mixture of comminutedquartz or silica mixed with a suitable binder such as fire clay, asabove described. This particular shape of the base layer 17 is to beconsidered as merely illustrative.

* Upon the support 17 is deposited successively the clear layer ofsilica 16 and a. translucent backing layer 18 by the burner 7 as abovedescribed.

It is to be understood, of course, that when either the article withthe' flat surface, as shown in Fig. 2, or with a shaped surface, asshown in Fig. 3, is completed, it is removed from the base or supportingsurface and is freed from clinging particles of the base by grinding andpolishing or by other suitable procedure. When the support consistseither of fire brick, as shown in Figs. 1 and 2, or of a mixture ofsilica and fire clay, as shown in Fig. 3, little difficulty isexperienced in cracking off the supporting layer from the silica. Thearticle finally may be trimmed, given a desired exact surfaceconfiguration and otherwise finished to a desired form. If desired, thelayer of clear silica may be increased in thickness by further sprayingof silica thereon.

What I claim as new and desire to secure by Letters Patent of the UnitedStates, is,

1. The process of making an article of vitreous refractory material bythe accretion of particles of said material at its fusion temperaturewhich consists in feeding said particles into a fusion zone during theformation of one part of said article at a rate at which a transparentdense product results and subsequently feeding particles of saidmaterial into a fusion zone during the formation of another part of saidarticle at a materia'lly higher rate whereby a less transparent and lessdense product is formed;

2. The process of making a vitreous silica article by theaccretion ofsilica particles with one .another at the fusion temperature of silicawhich consists in feeding said silica particles during the formation ofone part of said article at a rate at which the resulting product istransparent and dense. and subsequently feeding said silica particlesduring the formation of another part of said article at a materiallyhigher rate at which a product is formed which is of lesser density andtransparency.

3. The process of forming a silica article having a predeterminedconfiguration which con- .sists in forming a support having the desiredexternal configuration, depositing a layer of vitreous transparentsilica on said support by accretion, thereafter increasing the rate ofdeposition of silica to form a less dense layer of silica on saidtransparent layer, and finally removing the silica article thus madefrom the support.

4. The process of making an astronomical mirror of vitreous silica whichconsists in forming a support consisting of siliceous material and abonding agent, traversing the surface of said support with a flamecapable of fusing silica and projecting silica powder through said flameon said support at a rate at which a transparent layer of silica isformed on said support, subsequently increasing the rate of feed whilecontinuing said traverse to deposit on said transparent layer atranslucent, less dense layer of silica,

removing from said support the silica thus de ELIHU THOMSON.

